1. Field of the Invention
The present invention relates to subsurface stabilization techniques useful at hazardous waste sites or other excavation areas. More particularly, the present invention relates to methods and systems for achieving subsurface stabilization utilizing nonreplacement jet grouting techniques.
2. Relevant Technology
For about the last 40 years, the U.S. Department of Energy (DOE) has been producing hazardous and radioactive waste that is currently estimated at 4.5 million drums. Various proposals have been made for storage of hazardous and radioactive waste, such as transuranic waste, but there is still a large volume of buried waste material awaiting a safe storage solution.
Hazardous waste pits and other waste sites have come under scrutiny from the Environmental Protection Agency (EPA) because rain and surface water pass through the pits and leach hazardous materials into underground water supplies and otherwise contaminate surrounding environmental areas. Various techniques have been used or proposed in order to control contaminate migration from a buried waste site.
One of these techniques that has been used is termed in situ vitrification (ISV), in which joule heat is passed between electrodes, sufficient to melt the soil substrate and incorporated wastes into a relatively homogenous magma. The resultant melt solidifies to form a glass-like monolith, which can be removed in stone-like form and broken up essentially dust-free if retrieval is desired. The effectiveness of ISV is dependent on the amount of electrically conductive and glass forming materials in the soil/waste substrate.
The ISV technology is estimated to be significantly less expensive than other remediation technologies in remediating buried waste sites containing radioactive materials. Previous studies have shown that ISV, when applied to buried wastes, is 70-90% less expensive than standard "retrieve and treat" technologies. However, the application of ISV to buried waste sites has been hampered by a number of technical issues that have been often perceived as insurmountable.
The primary concern associated with ISV processing of buried wastes has been the presence of sealed containers filled with vaporizable material in the waste. The problem is that the materials in a sealed container will vaporize and build up pressure inside the container. The built-up pressure will suddenly be released from the sealed container when the pressure exceeds the yield stress of the container, causing a pressure surge in the vicinity of the molten melt front that can either cause the melt pool to be ejected from the site or significantly overwhelm the offgas system placed over the site.
Approaches that have been used to remove pressure build up in waste sites include the elimination of sealed container conditions from the waste prior to ISV processing. These approaches include the use of vibratory rods, and the preretrieval, visual sorting, and removal of sealed containers from the waste. These approaches, however, are fraught with contamination control problems, especially for buried transuranic waste scenarios.
The vibratory rod technique utilizes a large I-beam that is vibrationally inserted at designated spacings into the buried waste matrix, destroying adjacent packaging integrity, while compacting the surrounding soil. While the vibratory rod can collapse some large voids in the surrounding soils and destroy the integrity of sealed containers such as gas cylinders in the waste, a high level of soil disturbance occurs, which may result in contamination control problems. In addition, concerns exist about the high level of disturbance associated with fracturing compressed gas cylinders in situ. Other problems are that the energetic vibrations could possibly cause some buried waste materials to rise to the surface upon removing the I-beam from the soil once the vibration process is complete, especially if the amount of soil overburden is not sufficient, resulting in contaminant spread. In addition, the fractured containers in the buried waste matrix may re-seal and cause pressurization buildups, since the voids in these containers are not totally eliminated. Primary concerns with the preretrieval and visual sorting approach are that it eliminates many advantages associated with in situ treatment of the waste by introducing additional costs associated with preretrieval, waste sorting, and contamination control.
A dynamic compaction process has also been proposed as an ISV pretreatment method to collapse voids and destroy sealed container integrity. However, the dynamic compaction process is extremely energetic, and could result in contamination control concerns at many sites. A secondary concern associated with ISV processing of contaminated waste sites is the potential transfer of highly volatile contaminants into the soils surrounding the ISV melt, effectively transferring the contaminants to another location, rather than actually treating the contaminants in situ.
Another prior technique for controlling contaminant migration has been to utilize a vehicle with an arm that has jet grouting capabilities which is used to form underground walls, such as an underground bathtub-shaped wall around a waste pit for containment of waste materials to prevent horizontal migration. Such a technique is disclosed in U.S. Pat. No. 5,542,782 to Carter et al., which discusses an apparatus for cutting soil and constructing subsurface containment barriers, such as containment walls or basins around and under contaminated soils. An elongated beam includes a cutting assembly having a conduit containing a plurality of jet ports through which high pressure fluid is ejected to impact the soil to be cut. Subsurface containment barriers are formed by a jetted slurry or other suitable material which cuts and mixes with the soil. This technique, however, does not work well in hard soil areas.
Other approaches that have been used include the use of a vibratory rod or auger to make spaced apart holes in a waste pit area and thereafter filling the holes with grout. Such a technique is disclosed in U.S. Pat. No. 4,776,409 to Manchak, which discusses an apparatus that includes a power operated vehicle that supports a vertically movable frame that may be placed in contact with the upper surface of a hazardous waste impoundment. Cutter-injectors are rotated downwardly from the frame into the impoundment to form a vertically extending zone of particled material that is treated with a detoxifying agent that may be chemical, biological, or heat. The cutter-injectors are sequentially lowered into adjacent areas of the impoundment while rotating to homogenize the hazardous waste material therein to a desired depth. Thereafter, the cutter-injectors are ly moved upwardly while rotating, and simultaneously treatment chemicals for the hazardous waste material are injected therefrom. This system would not work, however, for radioactive waste control such as for plutonium waste since the cutter-injectors are exposed to air when withdrawn.
Also, it is known to put polyvinyl chloride (PVC) pipes in holes dug in a waste pit area and to pump grouting materials through the PVC pipe.
In each of the above techniques the jet grouting used forms a single point spray in each of the holes, leaving pockets in the treated area without any grout. This results in approximately 60% of the treated area not being stabilized.
Another approach to treating buried waste is disclosed in U.S. Pat. No. 4,981,394 to McLaren et al. This patent teaches a method for disposing of hazardous waste in which chemical components are added to waste materials in a landfill to create alkaline conditions that induce precipitation of calcite. This provides a cementing action around the waste which retards leachate formation and groundwater incursion.
Accordingly, there is a need for improved methods and systems for achieving subsurface stabilization at buried waste sites under a variety of conditions for safe storage of hazardous and/or radioactive wastes, as well as at other sites where excavation activities take place.
In addition, conventional retrieval operations for removing buried waste using conventional remote excavators for either full pit or hot spot retrieval can create considerable dust. The conventional contamination control systems offered to control dust spread and thus contaminate spread have been shown to have only a 70% control over dust spread. A 98% control over dust spread is required during transuranic waste retrieval operations. As a result, there is a need for methods and systems that enhance the control over dust spread in a radioactive environment waste retrieval operation.